8-Bicycloalkyl-CPFPX derivatives as potent and selective tools for in vivo imaging of the A1 adenosine receptor.

Imaging of the A1 adenosine receptor (A1R) by positron emission tomography (PET) with 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propyl-xanthine ([18F]CPFPX) has been widely used in preclinical and clinical studies. However, this radioligand suffers from rapid peripheral metabolism and subsequent accumulation of radiometabolites in the vascular compartment. In the present work, we prepared four derivatives of CPFPX by replacement of the cyclopentyl group with norbornane moieties. These derivatives were evaluated by competition binding studies, microsomal stability assays and LC-MS analysis of microsomal metabolites. In addition, the 18F-labeled isotopologue of 8-(1-norbornyl)-3-(3-fluoropropyl)-1-propylxanthine (1-NBX) as the most promising candidate was prepared by radiofluorination of the corresponding tosylate precursor and the resulting radioligand ([18F]1-NBX) was evaluated by permeability assays with Caco-2 cells and in vitro autoradiography in rat brain slices. Our results demonstrate that 1-NBX exhibits significantly improved A1R affinity and selectivity when compared to CPFPX and that it does not give rise to lipophilic metabolites expected to cross the blood-brain-barrier in microsomal assays. Furthermore, [18F]1-NBX showed a high passive permeability (Pc = 6.9 ± 2.9 × 10-5 cm/s) and in vitro autoradiography with this radioligand resulted in a distribution pattern matching A1R expression in the brain. Moreover, a low degree of non-specific binding (5%) was observed. Taken together, these findings identify [18F]1-NBX as a promising candidate for further preclinical evaluation as potential PET tracer for A1R imaging.

Radiosynthesis and In Vitro Evaluation of [11C]tozadenant as Adenosine A2A Receptor Radioligand.

Tozadenant (4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide) is a highly selective adenosine A2A receptor (A2AR) antagonist and a promising lead structure for the development of A2AR-selective positron emission tomography (PET) probes. Although several 18F-labelled tozadenant derivatives showed favorable in vitro properties, recent in vivo PET studies observed poor brain penetration and lower specific binding than anticipated from the in vitro data. While these findings might be attributable to the structural modification associated with 18F-labelling, they could also reflect inherent properties of the parent compound. However, PET studies with radioisotopologues of tozadenant to evaluate its cerebral pharmacokinetics and brain distribution are still lacking. In the present work, we applied N-Boc-O-desmethyltozadenant as a suitable precursor for the preparation of [O-methyl-11C]tozadenant ([11C]tozadenant) by O-methylation with [11C]methyl iodide followed by acidic deprotection. This approach afforded [11C]tozadenant in radiochemical yields of 18 ± 2%, with molar activities of 50-60 GBq/µmol (1300-1600 mCi/µmol) and radiochemical purities of 95 ± 3%. In addition, in vitro autoradiography in pig and rat brain slices demonstrated the expected striatal accumulation pattern and confirmed the A2AR specificity of the radioligand, making it a promising tool for in vivo PET studies on the cerebral pharmacokinetics and brain distribution of tozadenant.

[18F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging.

Selective inhibition of glycine transporter 1 (GlyT1) has emerged as a potential approach to alleviate N-methyl-d-aspartate receptor (NMDAR) hypofunction in patients with schizophrenia and cognitive decline. ALX5407 is a potent and selective inhibitor of GlyT1 derived from the metabolic intermediate sarcosine (N-methylglycine) that showed antipsychotic potential in a number of animal models. Whereas clinical application of ALX5407 is limited by adverse effects on motor performance and respiratory function, a suitably radiolabeled drug could represent a promising PET tracer for the visualization of GlyT1 in the brain. Herein, [18F]ALX5407 and the corresponding methyl ester, [18F]ALX5406, were prepared by alcohol-enhanced copper mediated radiofluorination and studied in vitro in rat brain slices and in vivo in normal rats. [18F]ALX5407 demonstrated accumulation consistent with the distribution of GlyT1 in in vitro autoradiographic studies but no brain uptake in µPET experiments in naïve rats. In contrast, the methyl ester [18F]ALX5406 rapidly entered the brain and was enzymatically transformed into [18F]ALX5407, resulting in a regional accumulation pattern consistent with GlyT1 specific binding. We conclude that [18F]ALX5406 is a promising and easily accessible PET probe for preclinical in vivo imaging of GlyT1 in the brain.

Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands.

Tracer development for positron emission tomography (PET) requires thorough evaluation of pharmacokinetics, metabolism, and dosimetry of candidate radioligands in preclinical animal studies. Since variations in pharmacokinetics and metabolism of a compound occur in different species, careful selection of a suitable model species is mandatory to obtain valid data. This study focuses on species differences in the in vitro metabolism of three xanthine-derived ligands for the A1 adenosine receptor (A1AR), which, in their 18F-labeled form, can be used to image A1AR via PET. In vitro intrinsic clearance and metabolite profiles of 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), an established A1AR-ligand, and two novel analogs, 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX), were determined in liver microsomes from humans and preclinical animal species. Molecular mechanisms leading to significant differences between human and animal metabolite profiles were also examined. The results revealed significant species differences regarding qualitative and quantitative aspects of microsomal metabolism. None of the tested animal species fully matched human microsomal metabolism of the three A1AR ligands. In conclusion, preclinical evaluation of xanthine-derived A1AR ligands should employ at least two animal species, preferably rodent and dog, to predict in vivo behavior in humans. Surprisingly, rhesus macaques appear unsuitable due to large differences in metabolic activity towards the test compounds.

Design, synthesis and biological evaluation of Tozadenant analogues as adenosine A2A receptor ligands.

With the aim to obtain potent adenosine A2A receptor (A2AR) ligands, a series of eighteen derivatives of 4-hydroxy-N-(4-methoxy-7-morpholin-4-yl-1,3-benzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide (SYN-115, Tozadenant) were designed and synthesized. The target compounds were obtained by a chemical building block principle that involved reaction of the appropriate aminobenzothiazole phenyl carbamates with either commercially available or readily synthesized functionalized piperidines. Their affinity and subtype selectivity with regard to human adenosine A1-and A2A receptors were determined using radioligand binding assays. Ki values for human A2AR ranged from 2.4 to 38 nM, with more than 120-fold selectivity over A1 receptors for all evaluated compounds except 13k which had a Ki of 361 nM and 18-fold selectivity. The most potent fluorine-containing derivatives 13e, 13g and 13l exhibited Ki values of 4.9 nM, 3.6 nM and 2.8 nM for the human A2AR. Interestingly, the corresponding values for rat A2AR were found to be four to five times higher. Their binding to A2AR was further confirmed by radiolabeling with 18F and in vitro autoradiography in rat brain slices, which showed almost exclusive striatal binding and complete displacement by the A2AR antagonist ZM 241385. We conclude that these compounds represent potential candidates for the visualization of the A2A receptor and open pathways to novel therapeutic treatments of neurodegenerative disorders or cancer.

Development and Evaluation of a Versatile Receptor-Ligand Binding Assay Using Cell Membrane Preparations Embedded in an Agarose Gel Matrix and Evaluation with the Human Adenosine A1 Receptor.

Guanosine-5'-triphosphate (GTP)-binding protein-coupled receptors are the target of up to 40% of prescribed medications worldwide. To evaluate the suitability of novel receptor ligands, frequently elaborate, time-consuming, and expensive receptor-ligand interaction studies have to be carried out. This work describes the development and proof of principle of a rapid, sensitive, and reliable receptor-ligand binding assay. CHO cells were stably transfected with a construct encoding the human A1 adenosine receptor (hA1AR). For ligand binding assays, membranes from these cells were prepared and embedded in low melting point agarose. These "immobilized" samples were incubated with tritiated 8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX), a well-established receptor antagonist. The KD and Bmax values as well as kinetic parameters (kon and koff) of receptor-ligand interaction were determined. Unspecific binding of various radiotracers to either the carrier material or the agarose gel matrix was negligible. The dissociation constant (KD) for [3H]DPCPX at the hA1AR was determined by saturation, competition binding, and kinetic experiments. These studies resulted in KD values of ∼3 nM, which is in good accordance with previously published data obtained from conventional receptor-ligand binding assays. The procedure described in this study simplifies classical binding studies to a kit-like assay. The receptors retained their binding properties even when preparations were dried completely. Transport and delivery of the material are conceivable without loss of biological activity. Therefore, other laboratories can perform binding studies without special equipment or the necessity to run a cell culture laboratory and/or to dissect tissue on their own.

Influence of binding affinity and blood plasma level on cerebral pharmacokinetics and PET imaging characteristics of two novel xanthine PET radioligands for the A1 adenosine receptor.

INTRODUCTION: The suitability of novel positron emission tomography (PET) radioligands for quantitative in vivo imaging is affected by various physicochemical and pharmacological parameters. In this study, the combined effect of binding affinity, lipophilicity, protein binding and blood plasma level on cerebral pharmacokinetics and PET imaging characteristics of three xanthine-derived A1 adenosine receptor (A1AR) radioligands was investigated in rats. METHODS: A comparative evaluation of two novel cyclobutyl-substituted xanthine derivatives, 8-cyclobutyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CBX) and 3-(3-[18F]fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine ([18F]MCBX), with the reference A1AR radioligand 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX) was conducted. This evaluation included in vitro competition binding assays, in vitro autoradiography and in vivo PET imaging. Differences in cerebral pharmacokinetics and minimal scan duration required for quantification of cerebral distribution volume (VT) were assessed. RESULTS: Measured Ki values of non-labeled CBX, MCBX and CPFPX were 10.0 ± 0.52 nM, 3.3 ± 0.30 nM and 1.4 ± 0.15 nM, respectively (n = 3-4). In vitro autoradiographic binding patterns in rat brain were comparable between the radioligands, as well as the fraction of non-specific binding (1.0-1.9%). In vivo cerebral pharmacokinetics of the novel cyclobutyl-substituted xanthines differed considerably from that of [18F]CPFPX. Brain uptake and VT of [18F]CBX were substantially lower despite the higher concentration of radiotracer in plasma. [18F]MCBX showed comparable uptake and VT, but faster cerebral kinetics than [18F]CPFPX. However, the faster kinetics of [18F]MCBX did not enable the quantification of cerebral VT in a shorter scan time. CONCLUSIONS: The combined effect of individual physicochemical and pharmacological properties of a radiotracer on its PET imaging characteristics cannot be readily predicted. In vivo performance of the xanthine A1AR radioligands was mainly influenced by binding affinity; plasma concentrations and cerebral kinetics were of secondary importance.

Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data.

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A1 adenosine receptor (A1AR). Using the half-life (t1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

Influence of incubation conditions on microsomal metabolism of xanthine-derived A1 adenosine receptor ligands.

INTRODUCTION: In vitro metabolism models such as liver microsomes represent an important tool for the development of novel radioligands. Comparability and physiological relevance of in vitro metabolism data critically depend on the careful evaluation and optimization of assay protocols. We therefore investigated the influence of incubation conditions on the microsomal stability of xanthine-derived A1 adenosine receptor (A1AR) ligands which have been developed for positron emission tomography (PET). METHODS: Substrate depletion assays using rat liver microsomes (RLM) were performed for three analogous compounds which differ with regard to the metabolically vulnerable substituent at the xanthine C8 position. Incubation conditions were varied systematically. Additionally, the stability of the cofactor NADPH during incubation was investigated. RESULTS: Microsomal metabolism was strongly influenced by buffer pH, organic solvents and preincubation time. Substrate depletion values varied up to 5-fold depending on incubation matrix composition, however, the rank order of metabolic stability remained unchanged. Prolonged incubation periods led to drastic loss in enzyme activity which could not be prevented by addition of metal chelators or antioxidants. Cofactor NADPH was rapidly oxidized in microsomal matrix, even in the absence of cytochrome P450 substrates. DISCUSSION: In summary, short incubation times, precise pH control and minimal concentrations of organic solvents are mandatory to obtain reliable microsomal stability data. Furthermore, in vitro metabolic stability of the tested A1AR ligands varied largely depending on the particular C8 substituent. Consequently, structural modifications at the xanthine C8 position appear to be a promising strategy for the improvement of A1AR PET radioligands.

Synthesis and Pharmacological Evaluation of Identified and Putative Metabolites of the A1 Adenosine Receptor Antagonist 8-Cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX).

The A1 adenosine receptor (A1 AR) antagonist [18 F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([18 F]CPFPX), used in imaging human brain A1 ARs by positron emission tomography (PET), is stable in the brain, but rapidly undergoes transformation into one major (3-(3-fluoropropyl)-8-(3-oxocyclopenten-1-yl)-1-propylxanthine, M1) and several minor metabolites in blood. This report describes the synthesis of putative metabolites of CPFPX as standards for the identification of those metabolites. Analysis by (radio)HPLC revealed that extracts of human liver microsomes incubated with no-carrier-added (n.c.a.)[18 F]CPFPX contain the major metabolite, M1, as well as radioactive metabolites corresponding to derivatives functionalized at the cyclopentyl moiety, but no N1-despropyl species or metabolites resulting from functionalization of the N3-fluoropropyl chain. The putative metabolites were found to displace the binding of [3 H]CPFPX to the A1 AR in pig brain cortex at Ki values between 1.9 and 380 nm and the binding of [3 H]ZM241385 to the A2A AR in pig striatum at Ki values >180 nm. One metabolite, a derivative functionalized at the ω-position of the N1-propyl chain, showed high affinity (Ki 2 nm) to and very good selectivity (>9000) for the A1 AR.

Efficient synthesis of [18 F]FPyME: A new approach for the preparation of maleimide-containing prosthetic groups for the conjugation with thiols.

An improved high yielding radiosynthesis of the known thiol-reactive maleimide-containing prosthetic group1-[3-(2-[18 F]fluoropyridine-3-yloxy)propyl]pyrrole-2,5-dione ([18 F]FPyME) is described. The target compound was obtained by a two-step one-pot procedure starting from a maleimide-containing nitro-precursor that was protected as a Diels-Alder adduct with 2,5-dimethylfurane. Nucleophilic radiofluorination followed by heat induced deprotection through a Retro Diels Alder reaction yielded, after chromatographic isolation, [18 F]FPyME with a radiochemical yield of 20% in about 60 min overall synthesis time. A variety of other [18 F]fluoropyridine based maleimide-containing prosthetic groups should be accessible via the described synthetic strategy.

New potent A1 adenosine receptor radioligands for positron emission tomography.

8-Cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX) is meanwhile an accepted receptor ligand to examine the A1 adenosine receptor (A1AR) in humans by positron emission tomography (PET). A major drawback of this compound is its rather fast metabolic degradation in vivo. Therefore two new xanthine derivatives, namely 8-cyclobutyl-1-cyclopropymethyl-3-(3-fluoropropyl)xanthine (CBCPM; 5) and 1-cyclopropylmethyl-3-(3-fluoropropyl)-8-(1-methylcyclobutyl)xanthine (CPMMCB; 6) were designed and synthesized as potential alternatives to CPFPX. In membrane binding studies both compounds showed nanomolar affinity for the A1AR. In vitro autoradiographic studies of [18F]5 and [18F]6, using rat brain slices, showed the expected accumulation in regions known to have a high adenosine A1 receptor expression while exhibiting the necessary low unspecific binding. However, in vitro metabolite studies using human liver microsomes revealed a comparable metabolic degradation rate for both new xanthine derivatives and CPFPX.

Collision-induced dissociation studies of caffeine in positive electrospray ionisation mass spectrometry using six deuterated isotopomers and one N1-ethylated homologue.

RATIONALE: In order to deepen the understanding of electrospray ionisation collision-induced dissociation (ESI-CID) fragmentation reactions of xanthine derivatives for the identification of metabolites using low-resolution liquid chromatography/mass spectrometry (LC/MS) analysis, basic experiments using caffeine (1,3,7-trimethylxanthine) as model compound have been performed. METHODS: Six deuterium isotopomers and one N1-ethylated homologue of caffeine have been synthesized and their ESI fragmentation spectra have been obtained by using LC/MS in combination with either standard or perdeuterated eluent mixtures. RESULTS: One result of these studies is the finding that the positive charges of the ESI-CID caffeine fragments are caused by the addition of protons. Furthermore, the performed experiments allow the determination of all molecular formulae of each ESI-CID caffeine fragment. CONCLUSIONS: As basic CID reactions of caffeine have been elucidated in this work, the developed fragmentation scheme may serve as a valuable tool for the interpretation of ESI-CID fragmentation spectra of more complex xanthine derivatives and their respective metabolites.

Synthesis of the main metabolite in human blood of the A1 adenosine receptor ligand [18F]CPFPX.

In human blood, the PET radiotracer [(18)F]CPFPX (1) is metabolized to numerous metabolites, one (M1) being the most prominent in plasma 30 min p.i. Because the mass of injected tracer is < or = 5 nmol, concentrations in plasma are too low to analyze. Human liver microsomes generate main metabolites having HPLC retention times identical to those in plasma. HPLC-MS tentatively identified M1 as 2. Synthesis of 2 and identical HPLC-MS spectra of 2 and M1 confirmed that assignment.

Autoradiographic comparison of in vitro binding characteristics of various tritiated adenosine A2A receptor ligands in rat, mouse and pig brain and first ex vivo results.

The adenosine A(2A) receptor in the basal ganglia is involved in the control of movement and plays a role in movement disorders such as Parkinsonism. Developing ligands to evaluate that receptor by noninvasive methods such as positron emission tomography has a high priority. In vitro radioligand binding guides the selection of ligands for in vivo application. This study measured the binding of the adenosine A(2A) receptor antagonist [(3)H]MSX-2 (3-(3-hydroxypropyl)-8-m-methoxystyryl)-7-methyl-1-propargylxanthine) to rat, mouse and pig brain by autoradiography. Other studies measured binding to membranes from PC12 pheochromocytoma cells. Those binding parameters were compared to those of the adenosine A(2A) receptor antagonist [(3)H]ZM241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino)ethyl)phenol), the adenosine A(2A) receptor agonist [(3)H]CGS21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosine) and the unselective adenosine receptor agonist [(3)H]NECA (5'N-ethylcarboxamido)adenosine). The potency order (K(d)) in the three species was [(3)H]ZM241385<[(3)H]MSX-2<[(3)H]NECA<[(3)H]CGS21680. The density of [(3)H]MSX-2 binding sites was greater in the striatum than in the cortex. Preliminary ex vivo experiments showed that by 10min after iv injection, [(3)H]MSX-2 and [(3)H]CGS21680 crossed the blood-brain barrier to the extent of almost 1% ID/g brain tissue, but [(3)H]NECA and [(3)H]ZM241385 to only 0.2% ID/g. The prior administration of unlabeled ZM241385 significantly lowered brain uptake of [(3)H]MSX-2. In conclusion, [(3)H]MSX-2 has a high affinity and sufficient selectivity for the adenosine A(2A) receptor. It penetrates the blood-brain barrier. Sensitivity to photoisomerization is a limitation. Further investigations assess its suitability as a ligand for imaging the brain adenosine A(2A) receptor.

Cerebral A1 adenosine receptors (A1AR) in liver cirrhosis.

PURPOSE: The cerebral mechanisms underlying hepatic encephalopathy (HE) are poorly understood. Adenosine, a neuromodulator that pre- and postsynaptically modulates neuronal excitability and release of classical neurotransmitters via A(1) adenosine receptors (A(1)AR), is likely to be involved. The present study investigates changes of cerebral A(1)AR binding in cirrhotic patients by means of positron emission tomography (PET) and [(18)F]CPFPX, a novel selective A(1)AR antagonist. METHODS: PET was performed in cirrhotic patients (n = 10) and healthy volunteers (n = 10). Quantification of in vivo receptor density was done by Logan's non-invasive graphical analysis (pons as reference region). The outcome parameter was the apparent binding potential (aBP, proportional to B (max)/K (D)). RESULTS: Cortical and subcortical regions showed lower A(1)AR binding in cirrhotic patients than in controls. The aBP changes reached statistical significance vs healthy controls (p < 0.05, U test with Bonferroni-Holm adjustment for multiple comparisons) in cingulate cortex (-50.0%), precentral gyrus (-40.9%), postcentral gyrus (-38.6%), insular cortex (-38.6%), thalamus (-32.9%), parietal cortex (-31.7%), frontal cortex (-28.6), lateral temporal cortex (-28.2%), orbitofrontal cortex (-27.9%), occipital cortex (-24.6), putamen (-22.7%) and mesial temporal lobe (-22.4%). CONCLUSION: Regional cerebral adenosinergic neuromodulation is heterogeneously altered in cirrhotic patients. The decrease of cerebral A(1)AR binding may further aggravate neurotransmitter imbalance at the synaptic cleft in cirrhosis and hepatic encephalopathy. Different pathomechanisms may account for these alterations including decrease of A(1)AR density or affinity, as well as blockade of the A(1)AR by endogenous adenosine or exogenous xanthines.

Metabolism of the A1 adenosine receptor PET ligand [18F]CPFPX by CYP1A2: implications for bolus/infusion PET studies.

The A1 adenosine receptor positron emission tomography (PET) ligand 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX, ) undergoes a fast hepatic metabolism. An optimal design of PET quantitation approaches (e.g., bolus/infusion studies) necessitates the knowledge of factors that influence this metabolism. Metabolites of were separated by radio thin-layer chromatography. Metabolism in vivo, in pooled human liver microsomes and in recombinant human cytochrome isoenzyme preparations was studied. Dynamic PET studies using were performed on three controls and two patients, one treated with the antidepressant and inhibitor of cytochrome CYP1A2 fluvoxamine, the other suffering from liver cirrhosis. CPFPX is metabolized by cytochrome CYP1A2 with high selectivity [KM=1.1 microM (95% confidence interval, or CI, 0.6-2.0 microM) and Vmax=243 pmol min(-1) mg(-1) (95% CI, 112-373 pmol min(-1) mg(-1)) corresponding to 2.4 pmol min(-1) pmol(-1) cytochrome P-450]. This metabolism can competitively be inhibited by fluvoxamine with KI=68 nM (95% CI, 34-138 nM). At least eight compounds found in human plasma and in the CYP1A2 in vitro preparations have an identical migration pattern and account together for >90% and >80% of the respective metabolite yield. Metabolism was considerably delayed in the two patients. In conclusion, is metabolized by cytochrome CYP1A2. Its metabolism is therefore subdued to disease-related or xenobiotic-induced changes of CYP1A2 activity. The identification of the metabolic pathway of 1 allows to optimize image quantification in A1 adenosine receptor PET studies.

Metabolism of the A(1)1 adenosine receptor positron emission tomography ligand [18F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([18F]CPFPX) in rodents and humans.

Studies of plasma from mice, rats, and human volunteers evaluated methods for the extraction and quantification of the positron emission tomography ligand [(18)F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([(18)F]CPFPX) and identification of its metabolites in plasma by thin-layer chromatography and high-performance liquid chromatography (HPLC). Analysis of human, mouse, and rat plasma extracts by HPLC identified four identical radioactive metabolites in each species. The low mass of radioligand administered to humans (0.5 - 5 nmol) prevented direct identification of metabolites. However, incubating liver microsomes with CPFPX and analysis by means of liquid chromatography-mass spectrometry (LC-MS) identified seven compounds, four having the same retention times as the metabolites in human plasma. Analysis of microsomal metabolites by LC-MS identified five [M + H](+) ions of m/z equivalent to hydroxy derivatives, 339, one of m/z equivalent to an oxo derivative, m/z 337, and one of m/z equivalent to a difunctionalized oxo-desaturation species, m/z 335, which is prominent in rat and mouse plasma and is the main metabolite in human plasma. An [M + H](+) ion corresponding to a N-dealkylated derivative was not detected. Thus, like the natural methylxanthines, CPFPX seems to undergo oxidation by liver microsomes but, unlike those methylxanthines, dealkylation did not occur. LC-MS experiments with "in source" fragmentation identified the cyclopentyl moiety to be the most functionalized part of the molecule by liver microsomes and in vivo oxidations. Except for two metabolites, hydroxylated at the N1 propyl chain, all oxidative modifications found took place at the cyclopentyl ring.

Synthesis and evaluation of 7-amino-2-(2(3)-furyl)-5-phenylethylamino-oxazolo[5,4-d]pyrimidines as potential A2A adenosine receptor antagonists for positron emission tomography (PET).

The brain A2A adenosine receptor (A2AAR) participates with the dopamine D2 receptor in the control of movement and also might influence behavior. Because PET is an important tool for studying the roles of receptors in disease, a ligand for imaging the brain A2AAR is desirable. This report describes the synthesis and A2AAR antagonist activities of a panel of phenyl-substituted 7-amino-2-(2-furyl)-5-phenylethylamino-oxazolo[5,4-d]pyrimidines, 11aa-af, and their 3-furyl congeners, 11ba-bd. In competitive binding studies all compounds displaced [3H]CGS21680 from the A2AAR with Ki values of 14-33 nM with selectivity for the A2AAR over the A1AR of 5- to 94-fold. Autoradiography of brain sections showed a high level of unspecific binding that obscured specific binding. Thus, these compounds are not promising PET ligands.

Derivatives of 4,6-diamino-1,2-dihydro-2-phenyl-1,2,4-triazolo[4,3-a]quinoxalin-2H-1-one: potential antagonist ligands for imaging the A2A adenosine receptor by positron emission tomography (PET).

The importance of the brain A2A adenosine receptor (A(2A)AR) in movement disorders urges the development of radiolabeled ligands for imaging those receptors by positron emission tomography (PET). This study evaluated one class of A(2A)AR antagonists, derivatives of 4-amino-6-benzylamino-1,2-dihydro-2-phenyl-1,2,4-triazolo[4,3-a]quinoxalin-2H-1-one, 10a, as agents for imaging brain A(2A)ARs by PET.. Modifications of a literature synthesis of 10a efficiently generated analogs 10b-s for pharmacological evaluation. Radioligand binding experiments showed affinities for the rat brain A(2A)AR in the low nanomolar range but similar affinities for the A1AR and substantial unspecific binding. Autoradiography employing [3H]10a, showing that high unspecific binding obscured specific binding to both the A1AR and A(2A)AR. Thus, compounds 10b-s are unsuitable as ligands for imaging brain A(2A)ARs by PET.